In a cellular access system such as a cellular telephony system, there is at least one cell, with one or more users in the cell, each of which user has a User Equipment, a UE, by means of which the user can communicate with the system. The system will also comprise at least one Radio Base Station, an RBS, which, inter alia, serves to control traffic to and from the UEs in the cell.
Thus, the UE and the RBS will communicate with each other, with the UE transmitting information to the RBS and receiving information from it, and likewise for the RBS with regard to the UE. In order to ensure that messages which are sent by one of the parties (UE/RBS) are received correctly by the receiving party (RBS/UE), a number of techniques are employed.
If the receiving party detects an error in a message that has been received from the transmitting party, or that a message has not been received at all, the receiving party can request a retransmission by the transmitting party, usually in the form of a NACK-message, Negative Acknowledge. In order to minimize the transmission delay it is essential that the receiving party can detect a transmission error as soon as possible. It is especially important to detect errors on retransmitted radio blocks or NACK-messages as soon as possible, since they correspond to already delayed radio blocks. A number of methods are used in order to achieve this, i.e. to detect erroneous retransmissions or NACK message errors:
One way to detect all kinds of transmission errors is to use a well protected header with a sequence number followed by separately coded data protected by an error detection code. If the header can be decoded, but the data is detected erroneous, the receiver can request a retransmission, i.e. send a NACK, for the block with the given sequence number.
It is also possible to use a timer for each retransmission, so that the receiving party starts a timer each time it requests a retransmission, i.e. sends a NACK. If the timer expires before the retransmission arrives, the receiver can conclude that either the NACK or the retransmission was erroneous, and a new NACK is sent.
However, with each of the methods described above, there are drawbacks, as illustrated in the following:
The method of detecting errors through error detecting codes requires the header to be separately decoded and correctly received, so that the sequence number of the lost packet is known. If the sequence number is not known, other methods are needed.
The method of using a timer for retransmissions is quite expensive, since all timers need to be maintained and controlled. In addition, it is difficult to obtain the correct value of a timer, since retransmissions may be delayed not only due to transmission errors, but also because of multi-user scheduling.